yup and also you mentioning the numbers being stamped into the metal hints at 3kpsi also as a 4.5k psi bottle made just of metal would be VERY heavy i would have thought, thats why we generally use fibre wrapped bottles to reduce the weight.
also general rule (with bottle used for paintball anyway) is if the bottle is flat bottomed its 3kpsi and if its round bottomed its 4.5k.....not a 100% rule but a good generalisation